When it comes to making mulled wine, the quality of the wine you choose may not be as important as when you're enjoying a glass on its own. This is because the intense spices and sweetness that you add to mulled wine will overshadow any subtle notes or complexities that a high-quality wine may have.
Let me share my personal experience as a sommelier and brewer. I have experimented with various types and qualities of wine when making mulled wine, and I have found that the nuances of a fine wine tend to get lost in the mix of flavors. Mulled wine is all about the warm, comforting spices and the sweetness, which provide the dominant taste profile.
That being said, this doesn't mean you should opt for the cheapest, lowest-quality wine you can find. A decent, drinkable wine is still important as the base for your mulled wine. You want a wine that has a good balance of acidity and fruitiness, as this will help complement the added spices and sugar.
In terms of specific wine recommendations, I would suggest looking for a medium-bodied red wine that is not too tannic. Avoid wines that are too light or too heavy, as they may not hold up well to the bold flavors of mulled wine. A Merlot, Cabernet Sauvignon, or Zinfandel can work well, but you don't need to splurge on high-end bottles. A mid-range wine will suffice.
If you prefer white mulled wine or want to try something different, a dry or semi-sweet white wine like Riesling or Gewürztraminer can be used. Again, you don't need to spend a fortune on these wines as their subtle characteristics will be overshadowed by the spices and sweetness.
While the quality of the wine is not the primary focus in mulled wine, it is still essential to choose a decent, drinkable wine that provides a good base for the flavors you'll be adding. Don't break the bank on expensive bottles, as the intense spices and sweetness will dominate the taste profile. Experiment with different wines to find your preferred balance of flavors, and enjoy the cozy warmth of mulled wine during the colder months.